In a fifth-generation mobile communication system, the frequency band to be used becomes wider, and therefore, radio waves in a high-frequency band, called millimeter waves, are expected to be used. The radio waves in a high-frequency band tend to attenuate. To counter this, there are suggested methods for improving communication quality between a base station and a mobile station by forming a high-gain directional beam using a directional antenna and compensate for the attenuation of radio waves. In such a system, the range in which communication can be performed with one directional beam is narrow, and therefore, communication is performed with a directional beam directed toward a mobile station. Hereinafter, a range in which communication can be performed with one directional beam will be referred to as a spot.
However, in a situation where a base station cannot identify the position of a mobile station, the base station cannot identify the direction in which a directional beam is to be directed. To counter this, a method called a beam sweep is used. By this method, a radio signal is transmitted in a plurality of directions while directions of directional beams are switched. The base station determines the direction of the directional beam so that a plurality of spots covers the entire area of the cell that is the communication area of the base station.
Patent Literature 1 discloses a mobile communication system that performs a beam sweep using a directional beam when a mobile station establishes the initial connection to a base station. The base station cyclically repeats transmitting a known synchronization signal and an annunciation signal containing information for connecting to the base station, and the mobile station detects the presence of the base station using the synchronization signal. Using the information contained in the received annunciation signal, the mobile station transmits a preamble to the detected base station, and starts a contention synchronization process. In order for the base station to receive the preamble transmitted from the mobile station using a directional beam, it is necessary to direct the directional beam in the direction in which the mobile station is located at the timing when the preamble reaches the base station. Therefore, in the system disclosed in Patent Literature 1, the mobile station is notified of the timing to direct a directional beam in the direction of the mobile station, through a synchronization signal or an annunciation signal. As the preamble is transmitted to the base station at the timing of which the mobile station is notified, it becomes possible for the base station to more certainly receive the preamble, and it is possible to shorten the time required until synchronization is established.
A situation where a base station cannot identify the position of a mobile station may occur not only during initial connection but also in a mobile station performing discontinuous reception (DRX) control. DRX control is a control method suggested for reducing power consumption by a mobile station in a mobile communication system such as Long Term Evolution (LTE), and the mobile station performs a receiving operation only for a short time during a cyclic reception period. If the mobile station moves and changes the distance to the base station after the base station has established the initial connection to the mobile station, the base station and the mobile station might fall out of synchronization. If data to be transmitted from the base station to the mobile station is generated in this situation, a synchronization process such as random access is performed to achieve synchronization. At this time, if the base station receives a signal from the mobile station, the base station can identify the direction of the mobile station. However, the timing of signal transmission from a mobile station performing DRX control is irregular, and therefore, the base station cannot identify the direction of the mobile station that is performing DRX control. In view of this, the base station might perform a synchronization process using a beam sweep.